Is it required by law or something for Russians to have a dash camera??

It is not required. It is rather a) not restricted by prohibitive laws, b) makes sense, since courts and insurance companies accept the footage as evidence.

Basically, people in Russia have the freedom to use dash cams, so they do use them. People in USA do not have this freedom.

People in USA would also use dash cams and the demand is pretty high (judging by the threads in this forum as well), but it is generally problematic. Firstly, it can land you in trouble if you accidentally film something you are not allowed to film: a cop or a government agent or something of that nature. The US regime is pretty sensitive to the danger of the citizens filming something they were not supposed to film. And, secondly, US courts will simply laugh at you if you attempt to use your footage as evidence. Empowering the citizens with the ability to produce hard evidence is not something the US regime can or would be ready to allow in the foreseeable future.

Basically, the freedom to use dash cams can be seen as a measure of general level of freedom and openness of the society. There is more personal freedom in Russia, hence the more dash cams.